Antiferromagnetic Resonance and Terahertz Continuum in α−RuCl3

Abstract

We report measurements of optical absorption in the zigzag antiferromagnet α-RuCl_{3} as a function of temperature T, magnetic field B, and photon energy ℏω in the range ∼0.3-8.3 meV, using time-domain terahertz spectroscopy. Polarized measurements show that threefold rotational symmetry is broken in the honeycomb plane from 2 to 300 K. We find a sharp absorption peak at 2.56 meV upon cooling below the Néel temperature of 7 K at B=0 that we identify as the magnetic-dipole excitation of a zero-wave-vector magnon, or antiferromagnetic resonance (AFMR). With the application of B, the AFMR broadens and shifts to a lower frequency as long-range magnetic order is lost in a manner consistent with transitioning to a spin-disordered phase. From a direct, internally calibrated measurement of the AFMR spectral weight, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum.

Magnetic field-dependent low-energy magnon dynamics in α–RuCl3

“…Consistent with the hypothesis of a magnetic origin of these resonances, we note that mode II was recently observed in independent TDTS [31][32][33]56] and EPR [34] experiments and assigned to a zone-center magnon of the zigzag ordered phase. On the other hand, while signatures of mode I have also been seen in previous measurements [28,31], this resonance has never been discussed. Specifically, both INS [28] and TDTS [31] spectra taken at different magnetic field amplitudes showed two distinct features at the zone center, similar to ours.…”

“…On the other hand, while signatures of mode I have also been seen in previous measurements [28,31], this resonance has never been discussed. Specifically, both INS [28] and TDTS [31] spectra taken at different magnetic field amplitudes showed two distinct features at the zone center, similar to ours. While both studies modeled the spectrum in terms of a single spin-wave peak, our extensive temperature and field dependence precludes this interpretation.…”

“…1(b)] [23,24]. Nevertheless, spectroscopic probes, including inelastic neutron scattering (INS) [25][26][27][28], spontaneous Raman scattering [29,30], time-domain terahertz spectroscopy (TDTS) [31][32][33], and electron paramagnetic resonance (EPR) [34], have discovered signatures of a field-induced QSL state above 7.5 T in the form of a broad continuum at the 2D magnetic Brillouin zone center. Yet a complete understanding of the origin of these excitations as well as of the spin dynamics is still lacking.…”

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…In α‐RuCl 3 , Ru 3+ () ions form almost ideal honeycomb lattice either in phase or in phase with almost no bond disproportionation ( l l / l s ≈ 1) (Figure ). At ambient pressure, α‐RuCl 3 exhibits a zigzag magnetic order below .…”

Pressure-induced dimerization and valence bond crystal formation in the Kitaev-Heisenberg magnet α−RuCl3

“…A structural phase transition was reported at T S 60 K under cooling and T S 166 K upon warming, but the low-temperature crystal structure is still under debate and could be either rhombohedral (R3) [10,11] or monoclinic (C2/m) [12,13]. The onset of long-range magnetic order at T N 7 K [9] in α-RuCl 3 implies that other magnetic interactions have to be considered in addition to the Kitaev interaction K: a NN Heisenberg J , an off-diagonal coupling , as well as next-NN interactions J 2 and J 3 [7,8,14,15].…”